Abstract

It is common practice to study physico-chemical phenomena by a procedure customarily referred to as variational spectroscopy. This approach entails the collection of spectra as some physical parameter is varied—for example, the temperature. Temperature-induced changes in the spectra are then monitored by measuring some spectral parameter, such as band position (or frequency), intensity, or width. Over the years we and others have used this type of measurement extensively to study temperature- and pressure-induced phase transitions of biological membranes. In the course of recent experiments on specifically labeled ¹³C=O phospholipids, we came across an unexpected and intriguing example of how one can be misled by a frequency shift.

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